1. Field of the Invention
The present invention relates to a solid-state imaging device, and more particularly to technology for reducing sensitivity shading.
2. Related Art
Solid-state imaging devices, remarkably popularized in recent years, typically receive external light via an optical lens.
FIG. 1 is a cross-sectional view of a digital camera illustrating the optical path of external light incident on a solid-state imaging device via an optical lens. As shown in FIG. 1, digital camera 1 is provided with an optical lens 101 and a solid-state imaging device 102, and external light 111 to 113 is incident on solid-state imaging device 102 via optical lens 101.
External light 112 indicated by the solid line is incident substantially at the middle of the imaging area of solid-state imaging device 102. In this case, the chief ray of external light 112 substantially coincides with the optical axis of optical lens 101, and is incident substantially vertically on the imaging area. In other words, the angle of incidence of external light 112 is zero.
On the other hand, external light 111 indicated by the broken line and external light 113 indicated by the dash-dotted line are, as shown in FIG. 1, incident at an angle at the periphery of the imaging area. In this case, the angle of incidence of the external light increases as the position of incidence moves away from the middle of the imaging area.
This produces a difference in light reception efficiency between pixel cells in the middle of the imaging area and pixel cells at the periphery, resulting in sensitivity shading. FIG. 2 is a cross-sectional view showing the structure of a typical pixel cell. As shown in FIG. 2, pixel cell 2 is provided with a semiconductor substrate 201, insulating films 202, 205 and 208, metal layers 203 and 206, a contact 204, a color filter 207, a collecting lens 209, and a photodiode 210 (see Japanese patent application publications No. 10-150182, and No. 2003-46865, for example).
External light incident on pixel cell 2 is incident on photodiode 210 via color filter 207 after having been focused by collecting lens 209. In this case, the external light must pass through metal layers 203 and 206 and the aperture of contact 204. However, when the angle of incidence is large, the incident light is shaded by metal layers 203 and 206 and contact 204. The incident light thus has difficulty reaching photodiode 210, resulting in sensitivity shading.
Metal oxide semiconductor (MOS) image sensors and charge coupled devices (CCDs) are known solid-state imaging devices. MOS image sensors are provided with a MOS-FET per pixel cell for amplifying the output charge of the photodiode, and thus require a plurality of metal wiring layers. Since this lengthens the optical path from collecting lens 209 to photodiode 210, sensitivity shading is particularly marked.